President Trump is threatening fresh tariffs on some of America's top trading partners.Tariffs are a surcharge placed on physical goods imported into the United States. President Donald Trump uses them to encourage companies to manufacture their products domestically rather than overseas. While this is great in theory, tariffs significantly increase prices for consumers, which can hurt economic growth, and they can also lead to a highly damaging global trade war.That's why the S&P 500 was down by as much as ...